Crawl space dehumidifiers and vapour barriers
These are the two smaller tools for a damp crawl space. A vapour barrier blocks moisture coming up from the soil. A dehumidifier takes moisture out of the air. Which you need depends on where the dampness is coming from.
Three levels of treatment
| Approach | What it does and when it fits |
|---|---|
| Ground cover only | Plastic over the soil, vents left open. Fits a crawl space that is mostly dry and simply has bare earth. It does nothing about humid air coming through the vents. |
| Ground cover plus dehumidifier | Sometimes offered as a middle step. With the vents open the machine is trying to dry the outdoors, so ask how the space will be closed off enough for it to cope. |
| Full encapsulation | Liner up the walls, vents sealed, dehumidifier inside. The complete answer to summer condensation. See how encapsulation is done. |
What a crawl space dehumidifier needs
- A closed space. It can only dry air that stays put. Open vents and a loose hatch defeat it.
- Power. A grounded outlet within reach, on a circuit that can carry it. Extension cords under a house are not an answer.
- A drain. The water it collects has to leave the crawl space, by a sloped line to the outside or by a small condensate pump.
- The right machine. A unit built for crawl spaces, sized to the space. A household dehumidifier from a hardware store is not made for the cool, dusty air under a house.
- Looking after. A filter to change and a drain line to keep clear. Ask how often and who does it.
Watch the humidity yourself
A wireless humidity sensor costs little and tells you more than a guess. Put the sensor under the house, keep the display in the kitchen, and look at it through a summer week. The U.S. Environmental Protection Agency advises keeping indoor humidity below 60 percent, and between 30 and 50 percent if possible.
If your crawl space sits well above that for days at a time, write the readings down and give them to the company. After any work, the same sensor shows you whether it is doing its job.
Before you agree to a dehumidifier
- Where will it drain? Ask to be shown the end of the line once it is in.
- What happens if it stops? Some units have an alarm or a remote display. Without one, a failure can go unnoticed for months.
- Is the electrical work included? A new outlet may need an electrician and a permit.
- What humidity are you aiming for, and how will I see it? Get the target in the written proposal.
Short answers
Can I lay a vapour barrier myself?
A simple ground cover is within reach of a careful homeowner if the crawl space is dry, tall enough to move in and clear of wiring hazards. Sealing walls, piers and vents is a different matter and is usually left to a company with the right tapes and fasteners.
Will a dehumidifier dry out a crawl space that floods?
No. It handles damp air. Water on the ground needs drainage and usually a pump.
Tell someone what you are seeing
Call and describe what is going on under the house or in the basement. You will be put through to an independent local waterproofing company that works in your part of Tuscaloosa County.
Calls may be recorded. The company pays this site for the introduction. See the referral disclosure.